I wish there was a definitive explanation of how this works. I don't think the above is exactly correct though, as I am roughy at 2825 and have a legendary that is rated 2812 and came with only one slot open. Not only that, but I have had several other weapons in the 1600-2800 range that have only one slot, many of them are well over 2000.
My clan mate just broke 3000, and he says he is still getting a max of two slots on his.
ego 3300 is when u will open the third slot..
As you progress, you hit stages where new/more bonus rolls spawn on guns. From what I've seen, this is also when they start spawning with less mod slots, but the number of slots does increase again.